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The carbon dioxide concentration in free air carbon dioxide enrichment (FACE) systems typically has rapid fluctuations. In our FACE system, power spectral analysis of CO concentration measured every second with an open path analyzer indicated peaks in variation with a period of about one minute. I used open-top chambers to expose cotton and wheat plants to either a constant elevated CO concentration of 180 μmol mol above that of outside ambient air, or to the same mean CO concentration, but with the CO enrichment cycling between about 30 and 330 μmol mol above the concentration of outside ambient air, with a period of one minute. Three short-term replicate plantings of cotton were grown in Beltsville, Maryland with these CO concentration treatments imposed for 27-day periods over two summers, and one winter wheat crop was grown from sowing to maturity. In cotton, leaf gas-exchange measurements of the continuously elevated treatment and the fluctuating treatment indicated that the fluctuating CO concentration treatment consistently resulted in substantial down-regulation of net photosynthetic rate ( ) and stomatal conductance ( ). Total shoot biomass of the vegetative cotton plants in the fluctuating CO concentration treatment averaged 30% less than in the constantly elevated CO concentration treatment at 27 days after planting. In winter wheat, leaf gas-exchange measurements also indicated that down-regulation of   and   occurred in flag leaves in the fluctuating CO concentration treatment, but the effect was not as consistent in other leaves, nor as severe as found in cotton. However, wheat grain yields were 12% less in the fluctuating CO concentration treatment compared with the constant elevated CO concentration treatment. Comparison with wheat yields in chambers without CO addition indicated a nonsignificant increase of 5% for the fluctuating elevated CO concentration treatment, and a significant increase of 19% for the constant elevated treatment. The results suggest that treatments with fluctuating elevated CO concentrations could underestimate plant growth at projected future atmospheric CO concentrations.

Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration